This is the third year of a three year project concerned with the organization of the somatosensory relay nuclei (primarily VPL) of the thalamus of the rhesus monkey. During the first two years of the project, using single neuron microelectrode recording, we determined the receptive field locations of a large number of thalamic neurons, and classified the neurons according to submodality type (e.g., cutaneous, joint rotation, deep pressure). These data were used to construct a "map" of the body as it is represented in the somatosensory thalamus. The map was found to have structural similarities to the body map on the postcentral gyrus. Horseradish peroxidase injections in the postcentral gyrus, labelling the thalamic neurons projecting to the area of the cortical injection, provided data that further substantiate the similarity between thalamic and cortical maps. During the current year, we plan to carry out further single neuron thalamic recordings, and will employ quantitative tactile stimulation (controlled-force skin indentation) in order to test the hypothesis that there are several distinct populations of VPL neurons, distinguishable from one another on the basis of their responses to such stimuli, and occupying identifiably different but overlapping regions of the somatosensory thalamus. BIBLIOGRAPHIC REFERENCES: Whitsel, B. L., Rustioni, A., Dreyer, D. A., Loe, P. R., and Allen, E. E. Thalamic projections to areas 3, 1, and 2 in macaque monkey. J. Comp. Neurol., 1977, in press. Loe, P. R., Whitsel, B.L., Dreyer, D.A., and Metz, C.B. Body representation in ventrobasal thalamus of macaque. A single unit analysis. In press, J. Neurophysiol., 1977.